The invention relates to an electric lamp with a glass lamp vessel which is closed in a vacuumtight manner and which has a longitudinal axis;
current conductors extending from the exterior into the lamp vessel; PA1 an electric element in the lamp vessel, connected to the current conductors, PA1 which lamp vessel has a seal on the longitudinal axis, through which seal at least one of the current conductors is passed, PA1 which at least one current conductor comprises a metal foil which is embedded in the seal and which lies substantially in a flat plane, PA1 an inner conductor being welded to said metal foil, extending into the lamp vessel and connected to the electric element, and an outer conductor being welded to said metal foil and issuing from the seal to the exterior, PA1 while the inner and the outer conductors each have an end within the seal, and on the metal foil (i) lie at a distance from one another seen in a direction transverse to the longitudinal axis and (ii) pass through one and the same axial zone of the seal.
Such a seal with such a current conductor passed through it is known from GB-B-512,257.
Current conductors comprising metal foils are widely used in seals when the glass of the seal has a coefficient of thermal expansion which is lower than the corresponding coefficient of the metal. This is the case if the glass must have a high softening temperature in view of the operational conditions of the lamp, while the metal for the same reason, and because of the high manufacturing temperature of the seal, must have a high melting point, such as tungsten and molybdenum.
The use of a metal foil means that the difference in coefficient of expansion between metal and glass, for example hard glass or glass having a SiO.sub.2 content of at least 95% by weight, such as, for example, quartz glass, does not detract from the vacuumtightness of the seal. A condition for this is, however, that the axial edges of the metal foil are sharp, i.e. the foil has axial knife edges (also called feathered edges).
An electric lamp having seals in which such foils with etched axial edges are enclosed is known, for example, from U.S. Pat. No. 4,851,733. Such seals are interesting because they can be manufactured quickly in a portion of a lamp vessel which is still tubular in that this portion is heated to the softening point and is flattened with pinching blocks for obtaining a pinch seal. It is true that metal wires can also be enclosed in glasses of a lower coefficient of expansion in a vacuumtight manner, as is known from U.S. Pat. No. 5,077,505 and U.S. Pat. No. 5,159,239, but in that case the wire must have previously been coated with a glass layer which must be fused to the glass of the seal circumferentially.
Such wires having glass layers have the advantage when used as current conductors that they can carry comparatively strong currents owing to their comparatively great cross-sectional areas, in contrast to metal foils. Pinch seals with metal foils, on the other hand, can be realized more quickly.
Metal foils can be enclosed in seals in a vacuumtight manner in spite of the differences in coefficient of thermal expansion provided they are comparatively thin, have a comparatively great width/thickness ratio, and have sharp axial edges. The sharp axial edges are necessary for achieving that the glass, which is comparatively viscous during making of the seal, comes into contact with the foil circumferentially the axis. Without sharp axial edges, a capillary channel would be formed along the axial edges of the foil, which always occurs along the transverse edges and around the inner and the outer conductor, which would mean that the lamp vessel is leaky right from the start.
To make the current density in a metal foil as small as possible, the foil may be given the greatest possible transverse dimension, but wide foils can reduce the resistance to pressure of the lamp vessel because the adhesion between glass and metal is usually smaller than the adhesion between glass and glass. In the electric lamp of DE-G-1 975 290, comparatively wide metal foils having knife edges along their axial sides, with several inner conductors being welded to the one axial end, are provided with a pattern of perforations for this reason. The glass at the one side of the foils is fused to the glass at the other side through the holes in the foils. The mechanical strength and resistance to pressure of the seal are increased thereby.
The current density in the metal foil of the lamp according to the cited DE-G is comparatively small for a given current owing to the width of the foil, and the current is passed into and from the foil over the entire width thereof owing to the plurality of inner and outer conductors, but the current path through the foil, which runs in axial lamp direction, is comparatively long, so that the foil still has a comparatively high resistance.
In the electric lamp of GB-A-489,626, several metal foils are arranged next to one another in a flat plane in an axial zone of each seal. This leads to a mechanically strong seal because the glass is fused on either side of the foils, but at the same time the current density in the foils is greater than if one foil were to occupy the width now occupied by the foils as shown in the drawing. In addition, the current path running in axial direction trough the foils is comparatively long.
In the seal described in the opening paragraph and conforming to the cited GB-B-512,257, both the outer and the inner conductor pass through substantially the entire length of the metal foil, at mutually opposed sides thereof, so that they overlap one another at a distance over an axial longitudinal portion of the seal. The current paths as a result run through the foil transversely to the longitudinal direction of the foil. A favorable aspect of this geometry is that there is a short and wide current path through the foil, so that the resistance of and the current density in the foil are comparatively small. A major disadvantage, however, is that this geometry is highly critical and involves a major risk of a leaky seal.
Metal foils are made in that pieces are cut off from a length of tape having sharp lateral edges. The cut edges are accordingly not feathered and sharp. The glass of the seal does not merge closely around the cut edges but leaves a capillary channel open which extends transversely along the foil in the seal. The inner or outer conductor runs over the relevant cut edge onto the foil. A capillary channel extends around the inner and around the external conductor to outside the seal because these conductors have a comparatively great thickness of several, for example, 7 or more tenths of a millimeter (in contrast to metal foils in seals which usually have a thickness of 10 to 120 .mu.m), and because they shrink more strongly than the surrounding glass after the seal has been made. These channels do not end until beyond the ends of the relevant conductors situated inside the seal.
The geometry of this construction involves the major risk that one or several of the capillary axial channels around the current conductors are in open connection with the two capillary transverse channels along the cut edges of the metal foil. The seal leaks in that case. It is in addition unfavorable that the conductors are welded along the axial, sharp edges of the foil where the foil is thin and a weld accordingly is mechanically very weak, which strongly limits the handling possibilities of the current conductor during lamp assembly. Another disadvantage is that the conductors are welded on either side of the metal foil in the known current conductor. This complicates the manufacture of the current conductor.
The seal shown in the cited GB-B-512,257, in which the inner conductor, the metal foil, and the outer conductor are stacked in an axial plane transverse to the seal, is of no use because it involves a very great risk of leaks. When welded joints are made between these metal parts, indeed, a hole may readily arise in the metal foil, affording access to either of the capillary channels around the two conductors owing to the geometry.